1. Field of the Invention
This invention relates to baking stones for use in commercial ovens and specifically to baking stones for ovens designed to cook pizza and bread. The improvement contributed by this invention is an asbestos-free formula for the manufacture of commercial baking stones and an improved baking stone made from the improved asbestos-free formula.
2. Description of the Related Art
The commercial oven industry was forever changed by the discovery that asbestos can be hazardous to human health. Baking stones used as oven decks in commercial ovens were made from materials broadly described as transite. Transite is a hydraulically pressed cement-asbestos board and was mass produced from the 1940's until 1985. Since the production of transite was discontinued, the commercial oven industry has continually searched for an equivalent baking stone which provides the same baking qualities as transite.
The quality most desired in a baking stone is even or consistent thermal conductivity through the stone. If the baking stone conducts heat unevenly, the bottom portion of food resting on the stone will not cook evenly. For example, a pizza crust may be burned in some areas and actually undercooked in other areas. Of course, the baking stone must be resistant to cracking and delamination despite years of use at temperatures exceeding 500.degree. F.
Transite was manufactured from a combination of portland cement and asbestos fibers. Manufacturers have continually searched for suitable fibers to combine with portland cement in order to achieve the desired baking qualities of transite. One such attempt is represented by the asbestos-free transite products. These products are manufactured from selected asbestos-free fibers and portland cement. The asbestos-free transite products are not a completely satisfactory replacement for asbestos-containing transite because of surface cracking, delamination and uneven thermal conductivity through the asbestos-free transite stone.
Another product which was developed to replace transite is sold under the name promassal. Promassal is a mixture of calcium silicate hydrates, calcium silicate fillers, phyllosilicate E fillers, natural organic fillers and residual quartz. However, promassal is characterized by the same inadequacies that characterize asbestos-free transite products. Namely, promassal baking stones are prone to surface cracking, delamination and uneven thermal conductivity through the stone. Specifically, promassal is not suitable for the higher temperatures used in baking pizza.
Another material commonly used for commercial oven baking stones is sold under the name cordierite. Cordierite is a magnesium-aluminum-silicate material. However, the cordierite stone is fragile and does not provide a consistent even cooking surface as required for hearth baked goods such as bread and pizza.
Other improved asbestos-free compositions for baking stones have been developed. For example, U.S. Pat. No. 5,334,242 discloses an improved composition for baking stones used in commercial ovens. The disclosed baking stone composition is asbestos-free, non-hazardous and is ideal for commercial ovens designed for baking pizza and bread. The improved composition includes portland cement, mortar or mason sand, glass fiber strands, water and a plasticizer.
However, in spite of the commercial availability of suitable asbestos-free baking stone compositions, there still exists a continuing need for improved asbestos-free baking stone compositions that produce a baking stone with greater thermal stability when subjected to numerous heating cycles in a commercial oven. It is well known that the thermal expansion of cement paste and aggregates differ. The effects of the difference in thermal expansion between cement paste and aggregates is very evident in baking stones which have been subject to heating cycles and temperature fluctuations in an oven. The continuous changes in oven temperatures often cause a significant break in the bond at the interface between the cement paste and the aggregates. Consequently, there has been a continuing need for an improved asbestos-free baking stone that provides for an extended service life even when the stone is subjected to heating cycles and temperature fluctuations in an oven.
Therefore, it is one object of the present invention to provide an improved asbestos-free baking stone that performs as well as or better than the asbestos-containing baking stones used in the past.
Another object of the present invention is to provide a baking stone that is highly resistant to surface cracking, highly resistant to delamination and provides even thermal conductivity through the stone.
Still another object of the present invention is to provide an improved asbestos-free baking stone that has greater thermal stability when subjected to numerous heating cycles in a commercial oven.